The invention relates to a flotation machine which consists of a flotation cell having a means for feeding slurry in the flotation cell, a gas dispersion mechanism for feeding gas into the slurry and producing froth, a means for removing froth from the flotation cell and a means for removing tailings from the flotation cell. According to the invention there is arranged at least one guiding device near the gas dispersion mechanism in order to guide the dispersed gas away from the gas dispersion mechanism.
A froth flotation machine for recovering valuable mineral material, normally with some hydrophobic surface, normally comprises a flotation cell in the form of a tank having an inlet in the cell wall for feeding slurry to be floated as well as an outlet for tailings in the lower part of the cell. Flotation cells may be single mixing vessels, in series or in parallel. They may be either rectangular or cylindrical in shape, in horizontal or upright position. Gas is supplied by some method to the gas dispersion mechanism, either through the hollow rotating shaft to the gas dispersion mechanism or by some independent supply line. The dispersion mechanism causes low pressure zones as well as high pressure zones as it rotates. In the gas dispersion mechanism the slurry in the flotation machine is mixed with the separately supplied gas, creating gas bubbles, which are discharged and dispersed into the whole space of the flotation cell. Often stationary baffles are installed around the gas dispersion mechanism to promote gas dispersion and attenuate the rotation of the slurry in the flotation cell. The valuable hydrophobic material in the cell attaches preferentially to the gas bubbles and rise to the surface of the cell, forming a froth layer and overflowing out of the cell into the froth launders.
Nowadays it is becoming increasingly common to use upright cells, which are also cylindrical and normally flat-bottomed. One problem with flotation cells is sanding, i.e. solid matter builds up on the bottom of the cell in an immovable layer. This is usually due to a too small or ineffective mixing and gas dispersion mechanism, as in such a case the mixing zone of the mechanism does not extend far enough. Another common difficulty is that the mineral particles already attached to the gas bubbles cannot be removed from the flotation cell, because the flows forming in the cell and particularly at its surface and upper section are wrongly oriented or too weak i.e. preventing loaded gas bubbles from entering the froth zone or froth exiting the cell.
It is also known before a flotation machine, a rotor of which has a plurality of vertical oriented plates, which form the pumping chambers. Air is pumped to each chamber via a vertical downcomer, which also incorporates and supports a horizontal shroud directly above the rotor. This shroud also supports the vertical stator blades. While the slurry flow entering the rotor is initially deflected upwards as it exits the rotor pumping slots it is deflected horizontally by the overhung shroud and is pumped radially outwards through the stator blades.
A significant zone of turbulence is created by the high velocity flows exiting the gas dispersion mechanism. This is the most important region within the flotation device for it is in this region that particle suspension, gas dispersion and gas bubble and particle contact takes place. If the flotation mechanism fails to properly disperse the gas into fine bubbles or contacts the gas bubbles with particles throughout the cell or if the solids suspension is inadequate then the flotation process suffers and the overall recovery of the desired material will be less. During the dispersion of gas by mechanical agitation, which creates negative pressure zone at the intake and positive pressure zone at the discharge, it is possible that gas, which has been supplied by some means to the gas dispersion mechanism short-circuits from the positive pressure zone back to the negative pressure zone occurs. This phenomenon is at times desirable during normal flotation practice but it can become excessive and counter productive as in the case of high viscosity or high density pulps. The re-circulation effect of gas into the lower pressure zone in the cell could be a problem with liquids either of high viscosity or heavy particle concentrations. This unnecessary circulation of dispersed gas within the gas dispersion mechanism reduces the overall efficiency and output of the gas dispersion mechanism.
According to a Finnish patent application 20001699 it is known a rotor used in the flotation mechanism in the concentration of ores. This type of rotor is developed especially for a material, which is coarse and has a high specific gravity. According to the invention of the application, upper part of the rotor chambers under the rotor cover are formed to be downward inclined from the outer edge of the chambers into the core so that they form an angle between 5 to 70 degrees with the horizontal plane. It has been proved that a vertical section downward tapered rotor which is equipped with means to direct the slurry flow upwards instead of horizontal direction allows the rotor to vary the mixing flow patterns within the machine and without interfering with the upper enrichment and froth zones within the cell. The invention relates also to a method to incline slurry flow essentially at the same angle as that of the means itself.
The object of the present invention is to eliminate the internal gas re-circulation existing, eliminate the drawbacks of the prior art, and to enhance the operation of flotation machine by preventing the gas dispersed by gas dispersion mechanism to flow back into the suction zone of the gas dispersion mechanism.
The essential features of the invention are enlisted in the appended claims.
According to the invention there is arranged at least one guiding device near the gas dispersion mechanism in order to guide the dispersed gas away from the gas dispersion mechanism. The device of the invention advantageously separates the input and output pressure zones of the gas dispersion mechanism and thus prevents gas re-circulation within the gas dispersion mechanism. By placing the guiding device between the pressure zone and suction zone of the gas dispersion mechanism, it guides the gas away and because of the natural buoyancy of the gas also upwards in the flotation machine, out of the reach of the rotor suction zone. The guiding device is placed advantageously so, that the return flow of the pulp is not disturbed. The size of the guiding device can be advantageously dimensioned so, that it matches with the flotation cell size, rotor size, gas dispersion mechanism design, different pulp viscosities and densities and other flotation parameters. According to one application of the invention the guiding device is arranged to extend essentially symmetrically around at least part of the gas dispersion mechanism. According to one application of the invention the guiding device is arranged to extend essentially symmetrically around the whole gas dispersion mechanism. Then it guides all of the gas dispersed around the gas dispersion mechanism upwards in the cell.
According to the invention the guiding device comprises of at least one wall element. According to one application of the invention at least part of the wall element is directed essentially vertically in the cell. According to one application of invention at least part of the wall element is directed essentially horisontally in the cell. According to one application of invention at least part of the wall element is placed so, that it forms an angle of 0-90 degrees with the horizontal plane. According to one application of invention at least part of the wall element is essentially straight. According to another application of invention the wall element is essentially curved. According to other application of invention the guiding device is essentially cone-shaped, opening upwards in the cell. Then the gas flow goes directly upwards in the cell. According to other application of invention the guiding device is made of light and wear resistant material. Advantageously the upper edge of the guiding device is located more near to the cell wall than the lower edge, which promotes the gas flow into right direction and leaves more space in the lower part of the cell. By preventing the re-circulation of flotation gas, also more flotation gas can be fed into the cell, which improves the overall performance of the flotation machine. The upward pulp flow in the flotation machine aids the upward gas flow and therefore further improves the operation of flotation process. The guiding device is easy to install and for example could be attached to the flotation machine by at least one fastening element like bolt.